1. Field of the Invention
This invention relates to an arrangement for preventing inductive interference in an electric car and more particularly to an arrangement for preventing inductive interference which is caused by switching currents used for operation of switching elements of an inverter in an inverter-driven electric car and which affects a high-frequency signal current for the electric car.
2. Description of Related Art
An electric car of the type as shown in FIGS. 11 and 12 has been known wherein an induction motor is driven by means of an inverter based on variable voltage/ variable frequency control. In the electric car designated at 48 in FIG. 12, DC power is fed from a feeder line 1 via a pantograph 10, the DC power is then supplied via breaker 12, filter reactor 14 and filter capacitor 16 to an inverter 18 at which the DC power is converted into AC power by means of switching elements 20, 22 and 24 constituting the upper arm and switching elements 26, 28 and 30 constituting the lower arm, and the AC power is applied to an induction motor 32 to drive the same. In the electric car 48 constructed as above, the inverter 18 is controlled with variable voltage/variable frequency to desirably adjust driving torque of the induction motor 32.
The electric car 48 using the inverter 18 however faces, as described in JP-A-62-123902, a problem that a high-frequency signal current, which is transmitted from a ground signal station to another signal station or the electric car through a signal line either directly or superimposed on a DC power current supplied from the feeder line 1, is subjected to inductive interference due to electromagnetic induction from the power line connecting the inverter 18 to the induction motor 32. For prevention of this problem, the power line is mounted as extending from the output of the inverter 18 along a car body of the electric car 48 or using twisted wires, while accommodated in a duct for shielding purpose.
The prior art simply relies on the manner of wiring the power line connecting the inverter 18 and induction motor 32 to eliminate the inductive interference and does not consider a stray capacitance 34 between a stator winding and a frame of the induction motor 32, thus failing to prevent the inductive interference satisfactorily.
More particularly, the frame of the induction motor is electrically connected to the wheel by way of the chassis and gear unit and an electric resistance across this path can be represented by a contact resistance 36 which totalizes contact resistance between gears included in the gear unit and contact resistance between each gear shaft and an associated bearing. The ground side of the inverter is electrically connected to a wheel 42 by return line 46 and ground brush 44 and drive current for driving the induction motor 32 flows through a closed circuit including a power supply (not shown), the feeder line 1, pantograph 10, inverter 18, return line 46, ground brush 44, wheel 42 and rail 40. Accordingly, in the presence of the stray capacitance 34 between the stator winding the frame of the induction motor 32, the stator winding is electrically connected to a wheel 38 through the stray capacitance 34 and contact resistance 36. Under this condition, when one of the switching elements of the upper arm, for example, switching element 24 is turned on, a loop of the switching element 24, stray capacitance 34, contact resistance 36, wheel 38, rail 40, wheel 42, ground brush 44, return line 46 and filter capacitor 16 is established as shown in FIGS. 11 and 12 to permit the switching current to flow through the rail 40. When one of the switching elements of the lower arm, for example, switching element 30 is turned on, a loop of the switching element 30, return line 46, ground brush 44, wheel 42, rail 40, wheel 38, contact resistance 36 and stray capacitance 34 is established as shown in FIGS. 13 and 14 to permit the switching current to flow through the rail 40. The switching current thus flowing through the rail 40 is a small but sufficient to inductively interfere the ground signal station which uses the rail 40 as signal transmission path.
To prevent the inductive interference due to the switching current of the inverter, JP-A-62-104401 and JP-A-62-114401 make proposals wherein there are provided a first ground circuit for connecting the ground side of the inverter to the wheel 42 via the ground brush 44 and a second ground circuit for grounding casings such as the motor frame and an enclosure box of the inverter to a different wheel from the wheel 42 via a different ground brush from the ground brush 44, and the first and second ground circuits are interconnected together by a by-pass capacitor. However, it has been found that even the above proposals have difficulties in sufficiently preventing the switching current from inductively interfering with a signal current recently used which has a high frequency up to, for example, 200 kHz.